1. Field of the Invention
This invention relates to a filter module, an optical module and a method for fabricating the same, and more particularly, to a filter module in which an optical filter fixed to a metallic base is assembled and an optical module in which an optoelectronic device is mounted on the filter module, each having a high temperature and high humidity resistance, and a method for fabricating the same.
2. Description of the Related Art
An optical filter assembly comprises multilayer filters bonded to a plurality of multilayer optical glass layers.
FIG. 4A is a schematic side view of a conventional optical filter assembly. The optical filter assembly 100 shown in FIG. 4A comprises a first multilayer filter 105, a second multilayer filter 106, a third multilayer filter 107, and a fourth multilayer filter 108 arranged in a single layer, and all optical wavelength reflecting film filter 109 opposed to the layer of the first to fourth multilayer filters 105 to 108. Herein, the first to fourth multilayer filters 105 to 108 are four kinds of the multilayer filters having different transmission wavelengths (or wavelength bands).
Herein, FIG. 4B is a schematic perspective view of a multilayer filter. Each of the first to fourth multilayer filters 105 to 108 is construed by forming a multilayer film 111 on one side of an optical glass plate 110 as shown in FIG. 4B.
In addition, the all optical wavelength reflecting film filter 109 is formed by providing a reflecting film 113 on one side of a first optical glass plate 112. On an extended line of the first to fourth multilayer filters 105 to 108 arranged in the single layer, second and third optical glass plates 114, 115 that having no special optical characteristic with respect to the optical wavelength are disposed. In concrete, the first to fourth multilayer filters 105 to 108 are sandwiched between the second and third optical glass plates 114 and 115. Further, another side of the first to fourth multilayer filters 105 to 108 arranged in the single layer is covered with a fourth optical glass plate 116. Namely, the single layer of the first to fourth multilayer filters 105 to 108 is covered with the all optical wavelength reflecting film filter 109 at its one side and covered with the fourth optical glass plate 116 at its opposite side.
In addition, an adhesive is used for bonding the respective optical glass plates each other. Of course, the adhesive is transparent with respect to the optical wavelengths.
In the optical filter assembly 100, when an optical wavelength multiplexed signal C (hereinafter referred as “light C”) is incident to a predetermined point of the fourth optical glass plate 116 with a predetermined incident angle as shown in FIG. 4A, the light C is transmitted through the fourth optical glass plate 116 and the second optical glass plate 114 and reflected back at the reflecting film 113 of the all optical wavelength reflecting film optical filter 109. The reflected light (i.e. a first reflected light) is incident on the first multilayer filter 105, then only a light having a specific transmission wavelength is transmitted through the first multilayer filter 105, and a light having other wavelength is reflected back at the multilayer film of the first multilayer filter 105. The reflected light (i.e. a second reflected light) is reflected back at a point which is advanced with respect to the reflecting point of the reflecting film 113 of the all optical wavelength reflecting film optical filter 109. When the reflected light (i.e. a third reflected light) is incident on the second multilayer filter 106, only a light having a specific transmission wavelength is transmitted through the second multilayer filter 106, and the light having other wavelength is reflected back at the multilayer film of the second multilayer filter 106.
As described above, the lights having different wavelengths are sequentially transmitted in the selective manner. Therefore, the optical wavelength multiplexed signal is divided into optical signals having optical wavelengths (or wavelength bands) different from each other. Further, if the optical signals having the optical wavelengths different from each other are incident to the optical filter assembly 100 in a reverse optical path, the optical signals having different optical wavelengths are multiplexed to provide an optical wavelength multiplexed signal. In other words, this optical filter assembly 100 may be used as an optical multiplexer-demultiplexer.
For example, Japanese Patent Laid-Open No. 2002-313140 (JP-A-2002-313140) discloses a conventional type transparent conductive film, optical filter and its manufacturing method.
In general, when optical components made of glass are incorporated in an optical communication apparatus such as optical transceiver, spectroscopic analyzer or other optical apparatuses, the glass made portion is not directly fixed to a housing of the optical apparatus. Normally, the optical component is previously fixed to a stay member to provide an optical assembly then the optical assembly is installed in the housing of the optical apparatus, thereby facilitating the assembling of the optical apparatus. Herein, it is easy and simple to use the adhesive to fix the optical components to the stay member of the housing.
However, according to a configuration in which the optical filter assembly is fixed by the adhesive to the stay member of the housing, there is a disadvantage in that the adhesive may be broken away in case where the optical apparatus is exposed to a high temperature and high humidity environment for a long time.
In the optical filter assembly per se, the adhesive used for bonding the respective optical glass plates may be exfoliated (i.e. broken away) under the above circumstances. If the moisture or air bubbles intrude into a bonding surface of the optical glass plate or a space between the bonding surface of the optical glass plate and the multilayer filter in accordance with the exfoliation (break away) of the adhesive, the optical characteristics of the optical multiplexer-demultiplexer will be fluctuated.
FIG. 5 is a schematic plan view of a part of the optical filter assembly 100 in a deteriorated state when viewed through an upper surface.
As shown in FIG. 5, when a single piece of the optical filter assembly 100 is observed after conducting the high temperature and high humidity test, it can be confirmed that bubbles 151 are intruded into the bonding surfaces between the first optical glass plate 112 and the second multilayer filter 106, and the fourth optical glass plate 116 and the second multilayer filter 106, respectively. Since the bubbles 151 are concentrated along side surfaces S1 that are contacting the edges of the bonding surfaces, it is presumed that the bubbles 51 are intruded from the side surfaces S1 into the bonding surfaces.